A synthetic resin guide L for a transmission device comprises an elongated rail 11 for longitudinal sliding engagement with a power transmission medium, and a rail supporting member 12 molded as a unit with the rail. The supporting member 12 comprises a plurality of reinforcing ribs 12c which support the rail 11. The ribs 12c are distributed along the length of the transmission device from a location adjacent one end of the rail 11 to a location adjacent the opposite end of the rail 11. The guide L is formed by injection molding and the reinforcing ribs 12c extend in directions following the flow of synthetic resin during injection molding. Preferably the synthetic resin is a glass fiber reinforced resin. By forming the reinforcing ribs 12c in this manner, high strength and toughness are imparted to the guide L, and warpage and torsion, which would otherwise be encountered in a high temperature transmission environment, are reduced.